DE69032044T3 - DIVERSITY RECEIVER FOR REMOTE CONTROL CELL SYSTEM WITH CODE MULTIPLEX MULTIPLE ACCESS - Google Patents

Abstract

A spread spectrum receiver subsystem for utilization in a CDMA cellular telephone having a searcher receiver for scanning the time domain so as to use the PN processing gain and time discrimination properties of spread spectrum coding to determine the location in the time domain and the received signal strength of multiple receptions of a pilot signal traveling upon one or more physical propagation paths to reception. The searcher receiver provides a control signal indicative of the received pilot signals of greatest strength and corresponding time relationship. A data receiver receives spread spectrum communication signals accompanying each received pilot signal and is responsive to the searcher control signal for acquiring and demodulating a spread spectrum communication signal, concomitant with the pilot signal of greatest signal strength, and thus providing a corresponding information bearing encoded output signal.

Description

Background of the invention

1. Field of the invention

The The present invention relates to cellular phone systems. Especially The present invention relates to a novel and improved receiver design for improving reliability and connections in the area of cellular phones.

2. Description of the related technology

The Use of CDMA Modulation Techniques (CDMA = Code Division Multiple Access = code division multiple access) is one of several techniques to facilitate communication, involving a large number of system users. Although other techniques, such as TDMA (Time Division Multiple Access), Frequency Division Multiple Access (FDMA) Frequency Division Multiple Access and AM modulation method, such as ACSSB (Amplitude Companded Single Sideband), CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Patent Serial No. 06 / 921,261, filed on October 17, 1986, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS ", now US Pat. No. 4,901,307, assigned to the assignee of the present invention Invention belongs and the disclosure of which is hereby incorporated by reference.

In the mentioned Patent discloses a multiple access technique in which a large number of mobile phone system users each have a transceiver (transceiver), which over Satellite relay stations or repeaters or terrestrial base stations (also known as Cell Stations) communicate using of CDMA spread spectrum communication signals. When using of CDMA transmission techniques the frequency spectrum can be reused several times, thereby an increase in system user capacity is allowed. The usage from CDMA gives a much higher spectral efficiency, as compared to other multiple access techniques can be achieved. Increases in system capacity can be realized in a CDMA system are controlled by controlling the transmitter power of each mobile user, to reduce interference with other system users.

at In the satellite application of CDMA communication techniques, the transceiver (transmitter / receiver) of the mobile unit, the power level of a received from a satellite relay station Signal. Using this power measurement along with knowledge the satellite transponder downlink transmission power level and the sensitivity of the receiver In the mobile unit, the transceiver of the mobile unit may track loss of the channel between the mobile unit and the satellite. Of the Mobile unit transceiver then determines the appropriate transmitter power, the for signal transmissions between the mobile unit and the satellite, namely considering path loss measurement, data transfer rate, and satellite receiver sensitivity.

The be transmitted from the mobile unit to the satellite signals from the satellite to an earth station of a central control system passed on. The central unit measures the received signal power from signals coming from each active Mobile unit transceiver transmit become. The center (hub) then determines the deviation of the received Achievement level of the one who is necessary to the desired Maintain connections. Preferably, the desired power level a minimum power level necessary to make good transfers maintain a reduction in system interference to surrender.

The Central (hub) then sends a power control command signal to each mobile user, the transmit power of the mobile unit to adjust or finely tune ("fine tune"). This command signal is issued by the Mobile unit used to change the transmission power level, and while closer to a minimum level required to achieve the desired levels Connections or transmissions maintain. If the channel conditions change, and typically due to movement of the mobile unit both mobile unit receiver power measurement also the power control feedback from the control center continuously changes the transmission power level, so a proper power level maintain. The power control feedback from the control panel is generally slow due to delays the signal path to the satellite and back, what about one 1/2 second runtime needed.

An important difference between satellite and terrestrial base station systems is the relative distances separating the mobile units and the satellite or cell station. Another important difference in the satellite system compared to the terrestrial system is the Type of fading or fading that occurs on these channels. Thus, these differences require various adjustments in system performance control for the terrestrial system.

in the Satellite / Mobile Unit Channel, i. the satellite channel, are the Satellite relay stations normally in a geostationary orbit arranged. Thus, the mobile units are all about the same same distance away from the satellite relay stations and therefore experience almost the same propagation loss. Further has the satellite channel has a propagation loss characteristic, the about the Inverse-square rule follows, d. H. the propagation loss is reversed proportional to the square of the distance between the mobile unit and the satellite relay station used. Corresponding to the Satellite channel the change path loss due to range changes typically in the Range of only 1-2 dB.

in the Unlike the satellite channel, the terrestrial / mobile unit channel, d. H. the terrestrial channel, the distance between the mobile units and the cell stations considerably vary. For example, a mobile unit may be at a distance of five Be located miles from the cell station while another mobile unit only a few feet away can be arranged remotely. The variation of the distance can exceed the factor of 100: 1. The terrestrial channel is subject to propagation loss characteristics as does the satellite channel. However corresponds to terrestrial Channel the propagation loss characteristic of a rule with inverse fourth power, i. the path loss is proportional to the inverse the path distance in the fourth power. Accordingly, path loss changes occur in the range of more than 80 dB, in one Cell with a radius of five Miles.

At the Satellite channel typically fades or fades, which can be characterized as Ricic. Corresponding the received signal is summed from a direct component with a multiply reflected component with Rayleigh fading or fading statistics. The performance ratio between the direct and the reflected component is typically of the order of magnitude of 6-10 dB, depending from the characteristics of the mobile unit antenna and the environment around the mobile unit.

at Confrontation of the satellite channel with the terrestrial channel occurs at the terrestrial channel signal fading, which is typically consists of the Rayleigh fading component without a direct component. Thus, the terrestrial channel provides a stronger fading environment than the Satellite channel where the Riconic fading has the dominant fading characteristic is.

The rayleigh Schwundcharakteristika the signal of the terrestrial Channels are caused by the fact that the signal from many different Properties or characteristics (objects) of the physical environment is reflected. As a result, the signal comes from many different ones Directions with different transmission delays almost simultaneously to a mobile unit receiver. In the ultra-high frequency bands, usually for mobile communications including the cellular Mobile phone systems can use significant phase differences occur on signals that follow different paths. The possibility Destructive addition of the signals may result, where occasionally Tiefschwünde can result.

Of the Weakness of the terrestrial channel is highly dependent on the physical position the mobile unit. A small change the position of the mobile unit alters the physical delays of all Signal propagation paths, which also has a different phase for each Path yields. For example, the movement of the mobile unit may be a relatively fast fading in the 850 MHz cellular radio frequency band, this fading typically up to one fade per second per mile can be vehicle speed per hour. A loss in this Magnitude can for Signals in the terrestrial channel be extremely disruptive, giving a poor communication quality results. However, additional transmitter power used to overcome the problem of shrinkage.

The terrestrial cellular A mobile phone system typically requires that a full-duplex channel be provided is to allow both directions the phone conversation are active at the same time, as is the case with conventional wired telephone system is provided. This full-duplex radio channel is usually provided by using a frequency band for the outward going connection, d. H. transfers from the cell-site transmitter to the mobile unit receivers. A different one Frequency band is used for the incoming connection, d. H. Transmissions from the mobile unit transmitters to the cell-site receivers. Accordingly, this frequency band separation allows a transmitter and receiver of one Mobile units are active at the same time without feedback or interference the transmitter with the receiver.

In the conventional cellular telephone system, the available frequency band is divided into Ka channels, which typically have a bandwidth of 30 KHz, while using analog frequency modulation techniques. The system operation area is divided geographically into cells of different sizes. The available frequency channels are divided into sets or groups, where each set or group usually contains an equal number of channels. The frequency sets are assigned to the cells such that the possibility of co-channel interference is minimized. For example, consider a system in which there are seven frequency sets and the cells are equal size hexagons. A frequency set used in a cell is not used in the six nearest or surrounding neighbors of that cell. Further, the frequency set in a cell is not used in the twelve closest neighbors of that cell.

At the usual cellular Telephone system should allow the implemented handover scheme that a call continues becomes when a mobile phone crosses the border between two cells. The handover from one cell to another is initiated when the cell-site receiver, which the conversation treated, noticed that the received signal strength from the mobile phone falls below a predetermined threshold. A low signal strength indicator means that Mobile phone must be near the cell boundary. When the signal level is below falls below the predetermined threshold, the cell station asks the system controller to determine if an adjacent cell site receives the mobile signal with a better signal strength than the current cell station.

The System controller sends in response to the current cell-site request Messages to the neighboring cell sites with a transfer request. The cell stations adjacent to the current cell site use special scanning receivers, on a special channel according to the signal from the mobile unit search. Should one of the neighboring cell stations of the system control unit indicate an appropriate signal level, then a handover tries.

The handover (Handoff) is initiated when empty or unused Channel selected from the channel set used in the new cell site. A control message is sent to the mobile phone and orders it switches from the current channel to the new channel. simultaneously the system control unit switches the call from the first cell station to the second cell station. In the conventional system, a so-called "break-before-make" scheme is used, so that no Diversity reception in case of overcoming Shrinkage possible is.

Should the mobile phone does not command to switch channels Listen, is beyond the handover fail. The actual Operating experience shows that unsuccessful handovers occur frequently what the reliability of the system in question.

At the usual cellular Phone system compromised a path fading (communication) and communication can be an interruption a conversation cause. It is therefore an object of the present invention to provide a cellular Telephone system a receiver construction providing for the reception and processing of the strongest, facilitates signals transmitted by one or more cell stations, these signals being more path signals from a single cell station are or are signals transmitted by multiple cell sites were.

Regarding the The prior art is also referred to US-A-4,291,410 which refers to a multipath spread spectrum communication receiver.

Summary the invention

This The goal is achieved by a recipient subsystem, as it is in the independent Claim 1, reached. In a cellular CDMA phone system will be the same frequency band for used the communication in all cells. The CDMA waveform properties, the one processing gain are also used to distinguish between signals, which have the same frequency band. Furthermore, the high-speed pseudonoise modulation allows (Pseudonoise = pseudonoise = PN) that many different propagation paths be distinguished, provided that the difference at the path propagation delays the PN chip duration or 1 / bandwidth exceeds. If a 1 MHz PN chip rate is used in a CDMA system, For example, the full spread spectrum processing gain equal to the ratio of the spread bandwidth to the system data rate is to be used opposite paths which is more than a microsecond of path distortion of the desired Distinguish path. A path delay difference of one Microseconds equals a path distance difference of 1000 feet. The urban Environment typically sees path delay differences of more as one microsecond, and up to 10 to 20 microseconds for some Areas reported.

Narrow-band modulation systems, such as the analog frequency modulation used in conventional telephone systems, result in the presence of multiple paths a strong multipath shrinkage. In broadband CDMA modulation, however, different paths can be distinguished in the demodulation process. This distinction severely prevents the extent of multipath fading. Multipath fading is typically not eliminated by using CDMA discrimination techniques because there are occasional paths with delay differences less than the minimum path delay for the particular system. Signals with path delays of this order can not be distinguished from each other in the demodulator. It is therefore desirable that the system provide diversity to further reduce the effects of fading.

The harmful Fading effects can occur in to some extent be controlled by controlling the transmitter power in a CDMA system. A system for power control of cell-site and mobile unit is disclosed in US Patent US-A-5 056,109, issued October 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM ", Serial No. 07 / 433,031, filed on 7 November 1989 by the same inventors and of the same applicant as the present invention. Furthermore, the Effect of multipath fading in the transfer mode (handoff mode) is reduced as the mobile unit transitions between cell station areas while the mobile unit is switching between cell station areas Mobile unit during the transfer process communicates with the cell stations. The transfer scheme is disclosed in US Pat. No. 5,101,501, issued Mar. 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM ", Ser. 07 / 433,030, filed on November 7, 1989 by the same Inventors and the assignee of the present invention.

The Presence of multipaths may provide path diversity for a wideband PN-CDMA system. When two or more paths are available with a path delay difference of more than one microsecond, two or more PN receivers can do so be used to receive these signals separately. This one Signals are typically related of multipath fading independently are, d. H. they usually disappear not together, can the expenses of the two recipients diversity-combined. Therefore, a loss of performance or function occurs only if both receivers fade at the same time. Therefore, one aspect of the present invention is that two or three more PN receiver in combination with a diversity combiner.

One Another aspect of the present invention is that when a mobile unit through the physical or physical environment moves, the number of multiple paths and their signal strengths constantly changes. The The present invention therefore uses a special receiver called Searcher receiver, which constantly scans or sweeps over the time domain of the channel, the presence, the location in the time domain, and the relative signal strength of signals in the multipath environment. The viewfinder receiver sees a control of the data receiver in which the best signals are sought on the different ones Paths available are.

at a cellular CDMA telephone system, each cell station has a variety of Modulator-demodulator units or spread spectrum modems. each Modem consists of a digital spread spectrum demodulator, at least one digital spread spectrum data receiver and a viewfinder receiver. Each modem at the cell site is a mobile unit as needed assigned to communicate with the associated mobile unit to facilitate. Therefore, many modems are in many cases for use available, while others can be active by communicating with respective mobile units. A soft transfer scheme (soft handoff) is for a cellular one CDMA telephone system used in which a new mobile station cell station modem is assigned while the old cell station maintains the connection or continues the call. If the mobile unit is in the transition area is arranged between two cell stations, the call can be between the cell stations are switched back and forth, depending on how it's the signal strength prescribes. Since the mobile unit always has at least one cell station communicates, no break effects on the mobile unit or occur at the connection. The present invention uses multiple Receiver in the mobile unit, which also uses a diversity function be, during the transfer process or stuck in a single cell.

at the cellular CDMA phone system transmits each Cell station a "pilot carrier" signal. This pilot signal is used by the mobile units to an initial System synchronization and provide a robust time, frequency, and phase tracking the signals transmitted by the cell station.

Each cell site also transmits a "setup" or "setup" channel consisting of spread spectrum modulated information, such as cell station identification, system timing, mobile call information, and various other control signals. The pilot signal transmitted from each cell station has the same spreading code but a different code phase offset. The phase shift allows that the pilot signals can be distinguished from one another, which results in a distinction between cell stations from which they originate. The use of the same pilot signal code allows the mobile unit to find a system timing synchronization based on a single search through all pilot signal code phases. The strongest pilot signal, as determined by a correlation process for each code phase, can be easily identified. The identified pilot signal corresponds to the pilot signal transmitted from the next cell station.

To Receiving the strongest Pilot signal, d. H. initial Synchronization of the mobile unit with the strongest pilot signal, seeks the Mobile unit to the appropriate device channel of this cell station. The facility channel is transmitted from the cell station using one of a plurality of different predetermined spread spectrum codes. In one embodiment The present invention uses 21 different codes. It should be noted, however, that more or less codes for The furnishing channel could be used, depending on how it went through the System parameter is determined. The mobile unit then starts one Search through all the different codes in the setup channel be used.

If the mobile unit will have the correct setup or setup code for it Cell station is identified, system information is received and processed. The mobile unit is monitoring continue the setup channel for control messages. Such a control message would show that a Call on the transmission waiting for this mobile unit.

The Mobile unit is running continue to receive the received pilot carrier signal code to scan at the code displacements, that of the neighboring Cell stations transmitted Correspond to pilot signals. This sampling is performed to determine whether the pilot signal originating from the neighboring cells stronger is considered the strongest first certain pilot signal. When in this mode with non-active connection or without a call, a pilot signal of a neighboring cell station stronger is considered that of the initial one Cell station transmitted Pilot signal, takes over the mobile unit the stronger Pilot signal and the corresponding device channel of the new cell station.

If a call or a conversation is initiated, a pseudonoise or PN code address is determined for use during this conversation or this connection. The code address can either be the cell station be assigned or may be predetermined based on the identity of the mobile unit be. After a call is initiated, the mobile unit continues to that transmitted by cell stations located in adjacent cells To scan the pilot signal. The sampling of the pilot signal is continued to determine whether one of the pilot signals transmitted by the neighboring cell sites stronger is, as the transmitted from the cell station pilot signal with the mobile unit communicates. If the pilot signal transmitted by a cell station, which is in a adjacent cell, becomes stronger than the pilot signal, transmitted by a cell station in the current cell is this is an ad for the mobile unit that in a new cell has been entered and that a handoff has been initiated should be. In response to this pilot signal strength determination creates and transmits the mobile unit a control message to the cell-site indicating the connection or the conversation served the time. This control message as an indication that one of a new cell station transmitted Pilot signal now stronger is as the transmitted from the current cell station pilot signal is delivered to the system controller. The control message also contains Information identifying the new cell station and the PN code. The control message delivered to the system controller becomes so interpreted that she means a handover the connection of the mobile unit to the identified new cell station imminent.

The system controller now begins the transfer process. It should be noted that during the handover, the PN code address of the particular mobile unit undergoing handover does not need to change. The system controller begins the handover by assigning a modem located in the new cell-site to the call. This modem is given the PN address associated with the call in the communication between the mobile unit and the current cell-site modem. The new cell-site modem that should pick up the call searches for and finds the signal transmitted by the mobile unit. The cell-site modem also begins to send an outgoing signal to the mobile unit. The mobile unit searches for this outgoing signal in accordance with the signaling and setup channel information provided by the new cell station. When the signal transmitted by the new cell-site modem is found, the mobile unit switches to listen for that signal. The mobile unit then sends a control message indicating that the handover is completed. The control message is delivered to the system controller by one or both of the old and new cell-station modems. In response to this control message, the system controller switches the call or connection to the new cell-site modem alone while the call or connection is being terminated or aborted via the old cell-site modem. The old cell-site modem then enters a pool of idle modems available for reassignment.

If however, the mobile unit within a single cell service area in which the cell station signals are multipath or multipath signals are the corresponding ones transmitted by the cell-site Signals stronger as those signals transmitted by any other cell station and were received by the mobile unit. Monitored in this single cell mode the search receiver the multipath signals and identifies the strongest, on the different multipath thread received signal. The search receiver provides this information to the mobile unit control processor, which the data receiver instructed to seek the signals on these strongest paths. The Signals are then output from the data receivers and sent to one Diversity combiner delivered.

During the Call Transfer are the mobile unit's connections to the different cell sites Exposed to path diversity. These compounds are also used by the multiple receivers processed in the mobile unit for diversity combination. Further become the signals sent by the different cell stations combined in a diversity combiner in the system controller. The present invention further allows what is referred to herein as cell station diversity mode is designated at other times than at a handover. In this mode, the mobile unit is allowed to come up with different ones Cell stations permanently connected.

In The cell station diversity mode is allowed to allow the call or the connection remains in an intermediate state, as above with respect to the processing of the call in two cell-sites has been described. In the embodiment, the herein with respect to the mobile phone of the present invention a total of three demodulator processors or receivers is used. One of these recipients will for the Scanning function used while the other two receivers used as a two-channel diversity receiver become. While of operation in a single cell, the sampling receiver attempts that of the Cell station transmitted Find signal that passes to the mobile unit on multiple paths. These multipath signals are typically caused by reflections of the signals through the terrain, buildings and causes other signal obstacles. If two or more such Reflections are found, the two receivers are the two strongest Associated with paths. The scan receiver continues the multiple paths evaluate to the two receivers with signals on the two strongest Keep paths synchronized as path conditions change.

In Cell Station Diversity mode becomes the strongest paths from each cell station through the search receiver certainly. The two receivers are associated with the demodulation of the signals on the two strongest paths, and though from the paths taken by the original cell station and the new cell station available are. The data demodulation process uses information from both of these recipients in a diversity combining operation. The result of this diversity combining operation is a greatly improved resistance to harmful fading in the Multipath environment of the cellular Telephone can occur.

The The present invention uses diversity combining to improve the quality and reliability of Compounds in a mobile, cellular telephone system significantly to improve. In the present invention, one kind of Maximum ratio combining used. The signal to noise ratio is determined for both paths, the be combined, with the contributions of both paths accordingly be weighted. The combination is coherent, as the pilot signal demodulation allows the Phase of each path.

In the path from the mobile unit to the two cell stations becomes Path Diversity Reception too obtained by both Cell stations demodulate the signals transmitted by the mobile unit. Both cell stations give their demodulated data signals to the System controller on, together with a display the signal quality at the cell-site receivers. The system controller then combines the two versions of the mobile unit signal and choose the signal with the best quality marks out. It should be noted that it possible is the non-decoded or even the non-demodulated signals to the system controller transferred to, to allow a better one Diversity combining process used becomes.

The System controller responds by placing the call with a Modem is connected in the new cell site. The system controller then performs a diversity combination of the signals transmitted by the two cell stations to be received while the mobile unit performs a diversity combination of the signals that received from the two cell stations. The cell or cell station diversity mode continues as long as signals are received from both cell sites were at a level sufficient to demodulate good quality to allow.

The Mobile unit is running continues to search for signals sent from other cell sites. When a signal sent from a third cell station becomes stronger as one of the original two cell-site signals, a control message is sent from the mobile unit via at least a current cell station is sent to the system controller. The control message indicates the identity of this cell site as well an application for surrender. The system controller then disconnects the connection via the weakest the three cell station signals exists while the connection or the Call over the two strongest Cell stations is provided. Should the mobile units with additional recipients equipped may be a triple cell station diversity mode, such as three receivers be implemented.

Of the Cell Station Diversity Mode is terminated when the mobile unit that determines only a cell station suitable signals for good quality demodulation supplies. The mobile unit then sends a control message that the cell station designates the connection after termination the cell station diversity mode should be maintained. The cell station diversity mode can also be used by the system controller will be stopped when the system overloads because there is an insufficient number of modems available, for all applications from mobile units to this mode or mode. Of the described cell station diversity mode is implemented by Decisions made in the mobile unit to be made in the mobile unit Cell Station Diversity mode to work. It should be noted, however, that cell station diversity mode can be implemented so that the decisions for the operation in this mode in the system control unit. It be noted also that the Cell station receiver use the multiple receiver architecture described above may provide for diversity reception when signals at the cell site arriving from a mobile unit after coming up with a larger delay difference as a PN chip.

The The present invention provides a substantial improvement over conventional ones cellular Telephone systems in terms of resistance or resistance across from Signal fading or fading by coherently combining multipath signals.

Short description the drawings

The Features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the drawings, in which the reference numerals are used the same throughout and in which:

1 Figure 3 is a schematic overview of an exemplary cellular CDMA telephone system according to the present invention;

2 Figure 3 is a block diagram of a mobile phone configured for CDMA connections in a CDMA cellular telephone system;

3 Figure 3 is a block diagram of cell station equipment in a CDMA cellular telephone system; and

4 is a block diagram of a mobile telephone switching center equipment.

Precise description the preferred embodiments

An exemplary telephone system embodying the present invention is disclosed in U.S. Patent Nos. 4,135,774, 5,348,059, 5,348,359, 4,395,359, and 5,348,934 1 shown. This in 1 The system illustrated uses CDMA modulation techniques in communication between the mobile units or cell phones and the cell stations of the system. Cellular systems in large cities can have hundreds of cell sites serving hundreds of thousands of cell phones. The use of CDMA techniques facilitates increases in user capacity in systems of this size compared to conventional cellular frequency modulation systems.

In 1 includes a system controller and central 10 , also referred to as Mobile Telephone Switching Office (MTSO), typically includes interface and processing circuitry for providing system control to the cell-sites. The control unit (controller) 10 controls the routing of telephone calls from the public telephone network (PSTN) to the corresponding cell station for transmission to the corresponding mobile unit. The control unit 10 also controls the piloting or routing of calls from the mobile units to the PSTN via at least one cell site. The control unit 10 can route calls between mobile users through the appropriate cell sites, as such mobile units typically do not communicate directly with each other.

The control unit 10 may be coupled to the cell sites by various means, such as dedicated telephone lines, optical fiber links, or microwave communication links. In 1 are two such exemplary cell stations 12 and 14 as well as mobile units 16 and 18 , each comprising a cellular telephone. arrows 20a - 20b and 22a - 22b each define the possible communication links between the cell station 12 and the mobile units 16 and 18 , Similarly define ren arrows 24a - 24b and arrows 26a - 26b in each case the possible communication connections between the cell station 14 and the mobile units 16 and 18 , The cell stations 12 and 14 send nominally using the same power.

The cell-site service areas or cells have geographical shapes so that the mobile unit is usually closest to a cell-site. If the mobile unit is idle, ie, no calls are made, then the mobile unit constantly monitors the pilot signal transmissions from each nearby cell station. As in 1 is shown, the pilot signals from each of the cell stations 12 and 14 via communication links 20a and 26a to the mobile unit 16 transfer. The mobile unit then determines which cell it is in by comparing the signal strength of the pilot signals transmitted by these particular cell sites.

The mobile unit 16 measures the total received power of the cell stations 12 and 14 over the paths 20a and 26a transmitted pilot signals. Similarly, the mobile unit measures 18 the total received power of the cell stations 12 and 14 over the paths 22a and 24a transmitted pilot signals. In each of the mobile units 16 and 18 the pilot signal power is measured in the receiver, the signal being a wideband signal. Accordingly, this power measurement is performed before the correlation of the received signal with a pseudonoise (PN) spread spectrum signal.

When the mobile unit 16 closer to the cell station 12 is, the received signal power is dominated by the signal that is on the path 20a emotional. When the mobile unit 16 closer to the cell station 14 is, the received power is dominated by the signal that is on the path 26a emotional. When the mobile unit 18 closer to the cell station 14 Similarly, the received power from the signal on the path will be similar 24a dominated. When the mobile unit 18 closer to the cell station 12 is, the received power is dominated by the signal that is on the path 22a emotional.

Each of the mobile units 16 and 18 uses the resulting measurement along with knowledge of the cell station transmitter power and the mobile unit antenna gain to estimate the path loss to the next cell station. The estimated path loss along with knowledge of the mobile antenna gain and cell site cell G / T (the receive antenna gain G shared by the receiver noise level T) is used to determine the nominal transmitter power required to obtain the desired signal to noise ratio in the cell-site receiver. The knowledge of the cell station parameters by the mobile units may be either fixed in memory or transmitted in cell station information transmit signals, such as in the setup channel, to indicate other than the nominal conditions for the particular cell station.

In the in 1 As shown, the mobile unit 16 as closest to the cell station 12 to be viewed as. When the mobile unit 16 initiating a call, a control message is sent to the next cell station, namely the cell station 12 transfer. Upon receipt of the call request message, the cell station notifies 12 the system controller 10 and transmits the called number. The system controller 10 then connects the call via the PSTN to the intended recipient.

If a call is initiated within the PSTN, the control unit transmits 10 the call information to all cell stations in the area. The cell stations in turn transmit a call message to the intended receiving mobile unit. When the mobile unit hears a call message, it responds with a control message, which is transmitted to the next cell station. This control message signals to the system controller that this particular cell station is in communication with the mobile unit. The control unit 10 then routes the call to the mobile unit via this cell site.

Should the mobile unit 16 out of the coverage area of the initial cell station 12 An attempt is made to continue the call by routing the call through another cell-site. During the transfer process, there are two different ways to initiate the transfer of the call or to transfer it via another cell station.

The first method, called the cell-site initiated method, is similar to the transition method used by the first generation, original, analog, cellular telephone systems currently in use. In the method initiated by the cell-site, the initial cell-site notices 12 that that is done by the mobile unit 16 transmitted signal has fallen below a certain threshold. The cell station 12 transmits a handover request to the system controller 10 , The control unit 10 forwards the request to all neighboring cell sites, including the cell site 14 , The request transmitted by the control unit includes information relating to the channel, including the PN code quence, that of the mobile unit 16 is used. The cell station 14 tunes a receiver to the channel used by the mobile unit and measures signal strength, typically using digital techniques. If the recipients of the cell station 14 report a stronger signal than the signal strength reported by the initial cell station, then a handover to that cell station is made.

The second method of initiating handover is called handover handover. The mobile unit is equipped with a search receiver that can be used to sample the pilot signal transmission from neighboring cell sites, in addition to performing other functions. If it is determined that a pilot signal the cell station 14 stronger than the pilot signal of the cell station 12 , sends the mobile unit 16 a control message to the current cell station 12 , This control message contains information that identifies the cell station with the greater signal strength, in addition to information requesting a handover to that cell station. The cell station 12 transmits this control message to the control unit 10 ,

The mobile handover procedure different advantages over the handover procedure initiated by the cell-site. The mobile unit recognizes changes in paths between itself and the various neighboring cell stations much sooner and with much less effort than the cell stations can. However, to perform a handover handover, each mobile unit must with a search receiver be provided to perform the scanning function. In the here described embodiment a mobile unit with CDMA communication capability, the search receiver has additional Functions that require its presence.

When the mobile unit 16 within the coverage or coverage area of the cell station 14 is such that the transmitted signals of the cell station 14 are the strongest, the search receiver uses the mobile unit 16 Maximum strength multipath signals for processing in the multiple data receivers.

Should the mobile unit 16 from the coverage area of the initial cell station 12 An attempt is made to continue the call by routing the call through another cell-site. In Cell Diversity mode, the call is routed through multiple cell sites. The use of the diversity receiver system of the present invention enables communication between the mobile unit 16 and the cell stations 12 . 14 and various other cell stations.

2 shows in block diagram form the mobile unit. The mobile unit comprises an antenna 30 via a diplexer or duplexer 32 with an analogue receiver 34 and a transmit power amplifier 36 connected is. The antenna 30 and the diplexer 32 are of conventional construction and allow simultaneous transmission and reception via a single antenna. The antenna 30 collects the transmitted signals and delivers them via the diplexer 32 to the analogue receiver 34 , The recipient 34 receives the RF signals, which are typically in the 850 MHz frequency band, from the diplexer 32 for amplification and frequency conversion into an intermediate frequency. This translation process is accomplished using a frequency synthesizer of conventional construction which allows the receiver to be tuned to any of the frequencies within the receive frequency band of the entire frequency band of the cellular telephone.

The IF signal is then passed through a surface acoustic wave (SAW) bandpass filter which, in the preferred embodiment, is a bandwidth from about 1.25 MHz owns. The characteristics of the SAW filter are chosen to match the waveform of the SAW filter Match signal transmitted by the cell station and which has been directly sequence spread spectrum modulated by a PN sequence clocked at a predetermined rate which is 1.25 MHz in the preferred embodiment. This clock rate is chosen that she is an integer multiple of a number of common data rates, such as for example 16 Kbps, 9.6 Kbps and 4.8 Kbps.

The recipient 34 also has a power control function for adjusting the transmission power of the mobile unit. The recipient 34 generates an analog power control signal to the transmit power control circuit 38 is delivered.

The recipient 34 is also provided with an analog-to-digital converter (A / D converter) (not shown) for converting the intermediate frequency signal into a digital signal, wherein the conversion in the preferred embodiment is at a clock rate of 9.216 MHz, which is exactly eight times the PN chip rate is. The digitized signal is provided to each of two or more signal processors or data receivers, one of which is a search receiver, the remaining receivers being data receivers.

In 2 becomes the digitized signal output from the receiver 34 to digital data receivers 40 and 42 and to the search receiver 44 delivered. It should be noted that low cost low cost mobile units could have only a single data receiver, while high power units could have two or more data receivers to allow for diversity reception.

The digitized IF signal may include the signals from many calls currently in progress along with the pilot carriers transmitted from the current and all neighboring cell sites. The function of the receiver 40 and 42 is to correlate the IF samples with the correct PN sequence. This correlation process provides a property known in the art as "processing gain," which enhances the signal-to-interference ratio of a signal that matches a particular PN sequence while not enhancing other signals , The correlation output is then detected synchronously using the pilot carrier from the next cell station as the carrier phase reference. The result of this sensing process is a sequence of coded data symbols.

A characteristic of the PN sequence used in the present invention is that discrimination of multipath signals is provided. If the signal arrives at the mobile receiver after it has traversed more than one path, there will be a difference in the reception time of the signals. This reception time difference corresponds to the distance difference divided by the speed of light. If this time difference exceeds one microsecond, then the correlation process will distinguish one of the paths from the other. The receiver can choose to track and receive the former or the latter path. If two receivers are provided, such as the receivers 40 and 42 , then two independent paths can be tracked and received in parallel.

The search receiver 44 under the control of the control processor 46 is provided for continuously scanning or sweeping the time range by the nominal time of a received pilot signal of the cell station in search of other multipath pilot signals from the same cell station or for pilot signals transmitted by another cell station. The recipient 44 measures the strength of each receive of a desired waveform that occurs at a different time than the nominal time. The recipient 44 compares the signal strength of the received signal. The recipient 44 provides a signal strength signal to the control processor 46 as an indication of the strongest signals and the relative time relationship.

The processor 46 provides control signals to digital data receivers 40 and 42 so that everyone processes a different one of the strongest signals. Occasionally, a pilot signal transmitted from another cell site will have a greater signal strength than the signal strength of the current cell station. The control processor 46 would then generate a control message for transmission to the system controller via the current cell station requesting handover of the call to the cell station corresponding to the stronger pilot signal. The receivers 40 and 42 therefore, can handle calls from two different cell sites.

The expenses of the recipients 40 and 42 are connected to a diversity combiner and decoder circuit 48 delivered. The diversity combiner circuit used in the circuit 48 is included, only performs a time adjustment of the two received signal streams for their adaptation or alignment and adds them. This addition process can be carried out by multiplying the two streams by a number corresponding to the relative signal strengths of the two streams. This operation can be considered as a maximum ratio diversity combiner. The resulting combined signal stream is then decoded using a forward current error detection decoder, which is also in the circuit 48 is included.

In the embodiment, convolutional coding is used. The convolutional coding has a constraint length of 9 and a code rate of 1/3, ie three encoded symbols are generated and transmitted for each transmitted information bit. The optimal decoder for this type of code is the soft decision Viterbi algorithm decoder design. The resulting decoded information bits are applied to the user-digital baseband circuitry 50 passed.

The baseband circuit 50 typically includes a digital vocoder (not shown). The baseband circuit 50 also serves as an interface to a telephone or other handset or any other type of peripheral device. The baseband circuit 50 is suitable for a variety of different vocoder designs. The baseband circuit 50 provides output information signals to the user in accordance with the information provided by the circuit 48 delivered there.

Analog voice signals from a user, which are typically provided by a telephone or handset, are input to the baseband circuitry 50 delivered. The baseband circuit 50 comprises an analog-to-digital (A / D) converter (not shown), which converts the analog signal into digital form (not shown). The digi The signal is sent to the digital vocoder, where it is coded. The vocoder output is supplied to a forward error correction coding circuit (not shown) for error correction. This encoded digitized speech signal is from the baseband circuit 50 to a transmit modulator 52 output.

The transmit modulator 52 modulates the coded signal onto a PN carrier signal whose PN sequence is selected in accordance with the assigned address function for the call. The PN sequence is from the control processor 46 determined from call setup information transmitted by the cell-site and from the receivers 40 and 42 is decoded. As an alternative, the control processor 46 determine the PN sequence by means of a previous setup or coordination with the cell station. The control processor 46 provides the PN sequence information to the transmit modulator 52 and to the recipients 40 and 42 for call decoding. The output of the transmit modulator 52 is applied to the transmission power control circuit 38 delivered. The signal transmission power is provided by that of the receiver 34 supplied analog power control signal controlled. Control bits are transmitted by the cell stations in the form of a power adjustment command and are provided by the data receivers 40 and 42 processed. The power adjustment command is issued by the control processor 46 used in setting the transmission power level of the mobile unit. In response to this command, the control processor generates 46 a digital power control signal sent to the circuit 38 is delivered. Further information about the relationship between the recipients 40 and 42 , the control processor 46 and the transmission power control circuit 38 are also further described in the above-mentioned pending patent application.

The transmission power control circuit 38 gives the power controlled modulated signal to the transmit power amplifier circuit 36 out. The circuit 36 amplifies the IF signal and converts it to radio frequency by mixing with a frequency synthesizer output signal that tunes the signal to the proper output frequency. The circuit 36 includes an amplifier that boosts power to a final output level. The intended transmission signal is from the circuit 36 to the diplexer 32 output. The diplexer 32 couples the signal to the antenna 30 for shipment to the cell stations.

The control processor 46 is also capable of generating control messages, such as cell diversity mode requests and cell station connection termination commands. These commands are sent to the transmit modulator 52 delivered for sending. The tax processor 46 speaks to the data recipients 40 . 42 and the viewfinder receiver 44 received data to make the decisions regarding handover and diversity combining.

3 shows in block diagram form an embodiment of the cell-site equipment. At the cell site, two receiver systems are used, each having a separate antenna and an analog receiver for space diversity reception. In each of the receiver systems, the signals are processed identically until the signals undergo a diversity combining process. The elements within the dashed lines correspond to the elements corresponding to the connections between the cell-site and a mobile unit. The outputs of the analog receivers are also supplied to other elements used in connections to other mobile units.

In 3 a first receiver system consists of an antenna 60 , an analog receiver 62 , a viewfinder receiver 64 and a digital data receiver 66 , This receiver system can also optionally have a digital data receiver 68 include. The second receiver system comprises an antenna 70 , an analog receiver 72 , a viewfinder receiver 64 and a digital data receiver 66 , A cell station control processor 78 is also used in signal processing and for handoff and diversity control. Both receiver systems are equipped with a diversity combiner and decoder circuit 50 coupled. A digital connection 82 is used with the cell demodulator 84 and the circuit 80 under the control of the control processor 78 Signals to and from the MTSO ( 4 ) transferred to.

From the antenna 60 received signals are sent to the analogue receiver 62 delivered. The received signals are from an amplifier in the receiver 62 amplified and transmitted to an intermediate frequency by mixing with a frequency synthesizer output signal. The intermediate frequency signals are bandpass filtered and digitized in a process identical to that described above with respect to the analog receiver of the mobile unit. The digitized intermediate frequency signals are sent to the digital data receiver 66 , the optional data receiver 68 and the viewfinder receiver 64 are each processed in a similar manner as described above with respect to the digital data receivers and the mobile unit's searcher receiver 2 has been described. However, the processing by the digital data receivers and the searcher receivers is different in various respects for the connection of the mobile to the cell station opposite the connection used between the cell-site and the mobile unit.

at the incoming connection or mobile unit connection to the cell station, the mobile unit transmits no pilot signal used in signal processing in the cell station for the purposes of coherent Reference can be used. Thus, the compound of the mobile unit to the cell station a scheme of non-coherent modulation and demodulation using 64-bit orthogonal signaling.

The viewfinder receiver 64 is again used to sample the time domain around the received signal to ensure that the associated digital data receiver 66 and optionally the data recipient 68 Track and process the strongest available time domain signal. This tracking process is identical to that described with respect to the mobile unit. The viewfinder receiver 64 provides a signal to the control processor 78 the cell station, which control signals to the digital data receiver 66 and 68 provides for selection of the correct or appropriate received signal for processing.

In the 64-orthogonal signaling method, the symbols transmitted by the mobile unit have one of 64 different possibilities. A 6-bit symbol is encoded in one of 26, ie 64, different binary sequences. The set of selected frequencies is known as Walsh functions. The optimal receive function for the Walsh function is the fast Hadamard transform (FHT = Fast Hadamard Transform). In the viewfinder receiver 64 and in the digital data receivers 66 and 68 For example, the input signal is correlated as described with respect to the mobile unit receivers, with the correlator output being provided to an FHT processor. The FHT processor generates a set of 64 coefficients for every six symbols. The 64 symbols are then multiplied by a weighting function generated in the receiver. The weighting function is related to the measured signal strength. The weighted data is then output to the diversity combiner and decoder circuitry 80 delivered.

The second receiver system processes the received signals in a similar manner as described with respect to the first receiver system of FIG 3 has been described. The weighted 64 symbols used by the receivers 66 and 76 are output to the diversity combiner and decoder circuit 80 delivered. The circuit 80 comprises an adder comprising the weighted 64 symbols from the receiver 66 to the weighted 64 symbols from the receiver 76 added. The resulting 64 coefficients are compared with each other to determine the largest coefficient. The size of the comparison result, along with the identity of the largest of the 64 coefficients, is used to determine a set of decoder weights and symbols for use within a Viterbi algorithm decoder which is included in the circuit 80 is implemented.

Of the Viterbi decoder preferably has a restriction length of 9 and a code rate of 1/2. The Viterbi decoder is used to determine the most likely information bit sequence. For each Vocoder data block, nominal 15 milliseconds of data, a signal quality estimate is obtained and as a mobile unit power setting command along with the Transfer data to the mobile unit. Further information about the generation of this quality estimate is in details in the above-referenced application. This quality estimate is the average signal to noise ratio over the 15 millisecond interval time.

In 3 can be the optional digital data receiver 68 for improved performance of the system. This additional data receiver, alone or in combination with additional receivers, may track and receive other possible delay paths of signals transmitted by the mobile unit. The structure and operation of this receiver is similar to that described with respect to the digital data receivers 66 and 76 has been described. The recipient 68 is used to obtain additional diversity modes or modes. Optional additional digital data receivers providing additional diversity modes are extremely useful in those cell sites located in densely populated urban areas where there are many possibilities for multipath signals.

Signals from the MTSO are transmitted via the digital connection 82 under the control of the control processor 78 supplied to or coupled to the appropriate transmit modulator. The transmit modulator 84 spread spectrum modulated in accordance with a predetermined spreading function provided by the control processor 78 has been assigned the data for transmission to the intended receiving mobile unit. The output of the transmit modulator 84 is applied to the transmission power control circuit 86 delivered where under the control of the control processor 78 the transmission power can be controlled. The output of the circuit 86 is applied to the transmission power amplifier circuit 88 delivered.

The circuit 88 includes a summer for summing the output of the transmit modulator 84 with the output from other cell station transmit modulators. The circuit 88 further comprises a summer for summing the pilot signal output from the pilot signal generator 90 with the summed transmit modulator output signals. The circuit 88 Also includes a digital-to-analog converter, a frequency conversion circuit and an amplifier for converting a digital signal into an analog signal or converting the intermediate frequency signals as output from the transmit modulators into a high frequency and amplifying the RF signal, respectively , The output from the circuit 88 is at an antenna 92 delivered where it is radiated to the mobile units within the operating range of the cell station.

The cell-site control processor 78 has the responsibility for assigning digital data receivers and modulators for a particular call. The control processor 78 Also monitors the progress of the call, the quality of the signals and initiates the call termination in case of signal loss. The cell station communicates via the connection 82 with the MTSO, to which it is coupled via a conventional telephone line, optical fibers or a microwave connection.

4 shows in block diagram form the equipment used in the MTSO. The MTSO typically includes a system controller or control processor 100 , a digital switch 102 , a diversity combiner 104 , a digital vocoder 106 and a digital switch 108 , Although not shown, additional diversity combiners and digital vocoders between the digital switches 102 and 108 be switched.

If the cell diversity mode is active, the call is from two cell sites edited so that signals with nominally the same information from more than one cell station arrive at the MTSO. Due to fading and interference on the incoming connection from the mobile unit to the cell stations however, the signal at a cell station may have better quality as the signal at another cell station.

The digital switch 102 is used to route the information stream corresponding to a given mobile unit from one or more cell stations to the diversity combiner 104 or the corresponding diversity combiner, as indicated by a signal from the system control processor 100 was determined. If the system is not in the cell diversity mode, the diversity combiner can 104 either bypassed or supplied with the same information at each input terminal.

A plurality of serially connected diversity combiners and vocoders are provided in nominal for each call to be processed in parallel. The diversity combiner 104 compares the signal quality indications accompanying the information bits from the two or more cell station signals. The diversity combiner 104 selects those bits which correspond to the highest quality cell station on a frame by frame basis of the information for output to the vocoder 106 ,

The vocoder 106 converts the format of the digitized voice signal to the standard 64 Kbps PCN phone format, analog format, or any other standard format. The resulting signals are from the vocoder 106 to the digital switch 108 transfer. Under the control of the system control processor 100 the call is routed to the PSTN.

Speech signals for the mobile unit coming from the PSTN are under the control of the system control processor 100 from the digital switch 108 supplied to a suitable digital vocoder, such as the vocoder 106 , The vocoder 106 encodes the incoming digitized speech signals and provides the resulting information bit stream directly to the digital switch 102 , The digital switch 102 conducts under the control of the system control processor 100 the encoded data to the cell station or cells stations with which the mobile unit communicates. When the mobile unit is in a handoff mode, communicating with multiple cell sites, or in a cell diversity mode, the digital switch heads 102 the call to the appropriate cell-sites for transmission by the appropriate cell-site transmitter to the intended receiving mobile unit. However, if the mobile unit only communicates with a single cell station and is not in a cell diversity mode, the signal is only passed to a single cell station.

The system control processor 100 sees a control of the digital switches 102 and 106 before, for the purpose of directing or directing data to and from the MTSO.

The system control processor 100 Also determines the assignment of calls to cell sites and vocoders in the MTSO. Further, the system control processor communicates 100 with each cell station control processor for the assignment of designated calls between the MTSO and the cell station and for the assignment of PN codes for the calls. It should also be noted that according to 4 the digital switches 102 and 106 when two separate switches are shown, this function However, can be performed by a single physical switching unit.

If the cell diversity mode is used, the mobile unit is using the viewfinder receiver, the strongest Multipath signal from each of the two cell stations to search and capture. The digital data receivers are from the viewfinder receiver and controlled by the control processor so that they have the strongest signals demodulate. If the number of recipients is less than that Number of cell-sites transmitting information in parallel, is a switching diversity capability possible. For example, if there is only one data recipient and two Send cell stations, monitored the viewfinder receiver the pilot signals from both cell stations and selects the strongest signal for the receiver for demodulation. In this embodiment, the choice with a frequency up to every vocoder frame, or about every 15 milliseconds become.

Claims (3)

A direct sequence spread spectrum receiver subsystem comprising: a control processor ( 46 ); Viewfinder ( 44 ) for receiving multipath signals corresponding to multipath propagations, each multipath propagation including a direct sequence spread spectrum information signal together with a pilot signal, the pilot signals from different cell stations having the same spreading code but a different predetermined code phase offset, and multipath signals received from the same cell station having a resulting time difference with respect to each other, for demodulating the received multipath signals, for determining for each received multipath signal a signal strength from the respective pilot signal through a correlation process for each code phase and a corresponding temporal relationship between the pilot signals in the received multipath signals, and for providing to the control processor ( 46 ) a seeker signal indicating the pilot signals with maximum signal strength and corresponding temporal relationship; and receiver means ( 40 . 42 ) for receiving and demodulating respective ones of the multipath signals having the largest signal strength, the receiver means ( 40 . 42 ) are responsive to control signals supplied by the control processor ( 46 ) for demodulating respective ones of the multipath signals corresponding to those of the pilot signals having the largest signal strength to provide an information signal, and the pilot signal having the largest signal strength is used as a carrier phase reference for synchronously detecting the respective multipath signal; and wherein the control processor ( 46 ) serves to control the searcher means ( 44 ) and the receiver means ( 40 . 42 and distinguish from each other pilot signals from different cell sites based on their different predetermined code phase offsets. The subsystem according to claim 1, wherein the receiver means ( 40 . 42 ) Combination agent ( 48 ) for coherently combining the demodulated ones of the multipath signals to provide the information signal. The subsystem of claim 2, wherein the information signal is in error correction coded format, and wherein the receiver means further comprises decoding means (16). 48 ) for error correction decoding the information signal.